The present invention relates to control systems of the type having a plurality of remotely located process control units connected together through a communications link and, more particularly, to a control system in which each of the remote units sequentially assumes supervisory communication control of the communication link.
Many system type industrial installations, for example, those related to industrial process-type manufacturing and electrical power generation, employ a large number of physically distributed controlled devices and associated sensors for effecting coordinated operation of the overall system. In the past, coordinated control of the various devices has been achieved by manual operation and various types of semi-automatic and automatic control systems including electro-magnetic relay systems, hardwired solid-state logic systems, and various types of computer control systems. The computer systems have included central systems in which the various sensors and controlled devices are connected to a central computer and distributed control systems in which computers at remote stations are connected to the controlled devices and the remote stations are connected over a communications link to one another. The successful functioning of the control system is vital to any industrial process, and, accordingly, distributed systems have generally been preferred over central systems because the failure of one of the remotely located control computers generally does not cause a system wide failure as in the case of the failure of the central computer in the central system. However, in many distributed computer systems, one of the remote stations or a specially designed control unit generally handles supervisory communication control of the communication link and, for these systems, failure of the communication link supervisor can lead to a system-wide failure.
Recently there have been developed systems in which none of the remote stations, called remotes, is assigned permanent supervisory control over the communications link. Instead, each of the remotes takes turns controlling the communications link and, while in control, each remote may transmit information to other remotes and make requests to other remotes for information and receive information back in response to these requests. One such system is disclosed in the patent to Michael E. Cope, Pat. No. 4,304,001. In the system of this patent, each remote is assigned a unique sequence number in a predetermined succession order with each remote unit assuming supervisory communication control of the communications link on a revolving basis in accordance with the remote's relative position in the succession order. When one of the remote stations is in control of the communications link and it is time for that remote to relinquish control over the communications link, it does so by transmitting a control message addressed to the next remote in the succession order. The next successive remote addressed by the control message then takes over the communications link. The remaining remote stations, other than the next successive remote, all set variable transfer monitor timers depending on their position in the succession order from the next successive remote. The further a given remote is from the next successive remote, the longer the time out interval that is set in the transfer monitor timer for the given remote. Each of the remotes is designed to assume control over the communications link if the time interval set by its transfer monitor timer expires before a preceding remote in the succession order assumes control. In this manner, if one or more of the remotes for any reason fails to take control over the communications link, the next functioning remote in the succession order will take over control.